In this article, we test Putnam’s claim that online interactions are unable to foster social capital by examining the formation of bridging and bonding social capital in online networks. Using Burt’s concepts of closure and brokerage as indicators, we observe networks formed through online interactions and test them against several theoretical models. We test Putnam’s claim using Twitter data from three events: the Occupy movement in 2011, the IF Campaign in 2013, and the Chilean Presidential Election of the same year. Our results provide the first evidence that online networks are able to produce the structural features of social capital. In the case of bonding social capital, online ties are more effective in forming close networks than theory predicts. However, bridging social capital is observed under certain conditions, for example, in the presence of organizations and professional brokers. This latter finding provides additional evidence for the argument that social capital follows similar patterns online and offline.